JP4647112B2 - 4-cycle gasoline engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a four-cycle gasoline engine having a compression ignition region in which an air-fuel mixture in a combustion chamber is ignited at multiple points by adiabatic compression.
[0002]
[Prior art]
As means for improving the thermal efficiency of a four-cycle engine, it is known to increase the specific heat ratio of the working gas by leaning the air-fuel mixture to improve the theoretical thermal efficiency. Further, by making the air-fuel mixture lean, even when the engine is operated with the same torque, more air is sucked into the engine, so that the pumping loss can be reduced.
[0003]
However, leaning of the air-fuel mixture has limitations due to prolonged combustion period and unstable combustion. Therefore, this limit is expanded by stratified combustion that collects the mixture around the spark plug while maintaining a stratified state by in-cylinder injection to ensure ignitability. Since the air-fuel mixture is concentrated, there is a problem that the combustion temperature increases and NOx tends to increase.
[0004]
On the other hand, a diesel engine has a high compression ratio and can substantially eliminate pump loss by making the air / fuel ratio lean, so it has high thermal efficiency, but because of diffusion combustion, the air utilization rate is low and the output is low. Emission may occur, and the exhaust gas characteristics are poor.
[0005]
Accordingly, as a means for solving such a problem, a compression ignition type engine has been proposed in which a gasoline mixture is ignited at multiple points by adiabatic compression without using an ignition plug. In order to realize compression ignition combustion, it is necessary to activate fresh air using high-temperature residual gas heat. One method is to advance the closing timing of the exhaust valve and open the intake valve. Is known to form a negative overlap period in which both valves close before and after exhaust top dead center, and trap residual gas in the combustion chamber from the second half of the exhaust stroke to the first half of the intake stroke. .
[0006]
For example, Japanese Patent Laid-Open No. 2000-64863 discloses a negative valve overlap period in which both the exhaust valve and the intake valve are closed before and after exhaust top dead center, and compression is performed by preloading the residual gas confined in the combustion chamber. A technique for promoting ignition is disclosed.
[0007]
In this prior art, during low load operation, the residual gas amount is increased by advancing the closing timing of the exhaust valve, and the residual gas is compressed by retarding the opening timing of the intake valve. The necessary work is collected to prevent a decrease in thermal efficiency.
[0008]
[Problems to be solved by the invention]
By the way, in the prior art described above, the negative valve overlap period before and after exhaust top dead center is controlled to improve the thermal efficiency. However, the thermal energy of the residual gas depends on the engine load. fluctuate. That is, the residual gas temperature during low-load operation is low and gradually increases as the operation shifts to medium-load operation.
[0009]
In the prior art described above, ignition is performed during the compression stroke using the thermal energy of the residual gas confined in the combustion chamber during the negative valve overlap period. It may become difficult to cause ignition failure without reaching the temperature, or conversely to cause early ignition, and it may be difficult to obtain stable compression ignition performance. As a result, there is a problem that the compression ignition region becomes narrow.
[0010]
In view of the above circumstances, an object of the present invention is to provide a four-cycle gasoline engine that can obtain a stable compression ignition performance in a wide region without being affected by load fluctuations in the compression ignition region .
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the present invention includes a variable valve mechanism capable of forming a negative valve overlap period in which both the exhaust valve and the intake valve are closed before and after exhaust top dead center. In a four-cycle gasoline engine partially having a compression ignition region, in the compression ignition region, the intake valve is closed so that the negative valve overlap period is narrowed and the volumetric efficiency is reduced as the engine load increases. It is characterized by advancing the time.
[0012]
In such a configuration, the thermal energy of the residual gas trapped during the negative valve overlap period is low during low-load operation and gradually increases as the engine shifts to medium-load operation, so that the engine load increases. , Advance the closing timing of the intake valve to reduce volumetric efficiency and reduce the negative valve overlap period to reduce intake air heating, thereby suppressing self-ignition and compression in a region where the air-fuel ratio is deeper Ignition is possible.
[0013]
In this case, preferably, the closing timing of the intake valve during the medium load operation is set in the vicinity of the intake bottom dead center.
[0014]
2) The closing timing of the intake valve during low load operation is retarded to a position beyond the intake bottom dead center.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an overall configuration diagram of a four-cycle gasoline engine having a compression ignition region .
[0016]
In the figure, reference numeral 1 is an engine body, 2 is a piston, 3 is a combustion chamber, 4 is an intake port, 5 is an exhaust port, 6 is an intake valve, 7 is an exhaust valve, and is connected to an intake passage 8 communicating with the intake port 4. A throttle valve 9 is interposed. The throttle valve 9 is connected to an electronically controlled throttle device (not shown) that electronically controls the throttle opening.
[0017]
In addition, an injection hole of an in-cylinder injector 11 as a fuel injection means faces the center of the top surface of the combustion chamber 3, and the top surface of the piston 2 facing the injection direction of the in-cylinder injector 11. A curved concave piston cavity 2a is formed.
Further, the ignition portion of the spark plug 12 is exposed to one side of the combustion chamber 3 (squish area in the present embodiment).
[0018]
An intake valve 6 and an exhaust valve 7 are connected to the variable valve mechanisms 13a and 13b, respectively. In the present embodiment, each of the variable valve mechanisms 13a and 13b includes two cams, a spark ignition intake cam and a compression ignition intake cam, and a spark ignition exhaust cam and a compression ignition exhaust cam. The cams are switched according to the operation region.
[0019]
Further, an intake cam for compression ignition and an exhaust cam for compression ignition are connected to a variable valve timing (VVT) mechanism. This VVT mechanism variably sets the valve timing in accordance with the engine load Lo by changing the rotation phase of the compression ignition intake cam and the compression ignition exhaust cam by an actuator such as a hydraulic solenoid (or electromagnetic solenoid). is there. Reference numeral 16 is a knock sensor, 17 is a water temperature sensor, and 18 is an O2 sensor.
[0020]
Signals detected by these sensors are input to an electronic control unit (ECU) 20. The electronic control unit (ECU) 20 is mainly composed of a microcomputer including a CPU 21, ROM 22, RAM 23, input port 24, output port 25, etc., which are connected to each other by a bidirectional bus 26.
[0021]
In addition to the above sensors, a crank angle sensor 31 that generates a crank pulse for each set crank angle is connected to the input port 24, and a load sensor 33 that generates an output voltage proportional to the amount of depression of the accelerator pedal 32. Are connected via an A / D converter 34. The output port 25 is individually connected to each variable valve mechanism 13a, 13b via an intake valve drive circuit 36a and an exhaust valve drive circuit 36b. Further, the in-cylinder injector 11 and the spark plug 12 are connected to the output port 25 via a drive circuit (not shown).
[0022]
The electronic control unit (ECU) 20 has a compression ignition region based on the engine speed Ne calculated based on the signal from the crank angle sensor 31 and the engine load Lo detected based on the signal from the load sensor 33. Or in the spark ignition region. When in the compression ignition region, the throttle valve 9 is fully opened (see FIG. 5 (d)), and the fuel injection amount that can obtain the optimal compression ignition combustion, The injection timing and the valve timing of the intake / exhaust valves 6 and 7 are set. When the operation area is in the spark ignition area, normal spark ignition control is executed.
[0023]
In order to obtain the optimum compression ignition combustion in the compression ignition region, the temperature of the fresh air sucked in the intake stroke is heated by the heat energy of the residual gas, and the residual gas and the fresh air are insulated in the compression stroke. It is necessary to raise the temperature to a temperature at which compression ignition is possible by compression. However, the thermal energy of the residual gas varies depending on the engine load, is low during low-load operation, and increases as the operation shifts to medium-load operation.
[0024]
Therefore, when performing the compression ignition control, the valve timing is continuously changed according to the engine load, and the engine load is operated at a low load as shown in FIGS. 4 (a), 4 (b), and 5 (c). The valve closing timing EVC of the exhaust valve 7 is gradually retarded and the valve opening timing IVO of the intake valve 5 is gradually advanced as the shift from the medium to the medium load operation direction, thereby reducing the negative valve overlap period. It is controlled so that it is wide on the side and narrows as it moves in the middle load direction.
[0025]
As a result, the amount of residual gas confined in the combustion chamber 3 increases on the low load operation side and decreases as the vehicle moves in the medium load operation direction. By increasing the amount of residual gas on the low load operation side, the temperature of the air-fuel mixture is increased by the heat energy of the residual gas, and compression ignition at a leaner air-fuel ratio becomes possible. On the other hand, by reducing the amount of residual gas as the vehicle moves in the medium load operation direction, self-ignition is suppressed and occurrence of knocking is avoided.
[0026]
In this case, if the closing timing of the intake valve 6 is constant, the region where compression ignition is possible becomes narrow. For example, as shown in FIG. 4A, when the valve closing timing IVC of the intake valve 6 is fixed at a position exceeding the intake bottom dead center (BDC), the low load region surrounded by the hatching shown in FIG. Compression ignition combustion is possible only in a narrow range. This is because when the negative valve overlap period is controlled and compression ignition combustion is performed using the thermal energy of the residual gas, the thermal energy of the residual gas increases as it moves toward the middle load direction. This is because knocking or the like easily occurs.
[0027]
Therefore, in the present embodiment, in the compression ignition region, not only the negative valve overlap period is variably set according to the engine load Lo to control the mixture temperature but also the closing timing IVC of the intake valve 6. Is also variably set to control volumetric efficiency so as to obtain appropriate compression ignition combustion.
[0028]
Specifically, the closing timing of the intake valve 6 is set in the combustion control routine shown in FIG. In this routine, first, in step S1, based on the engine speed Ne and the engine load Lo, referring to the operation region map shown in FIG. 3, whether the operation region is in the compression ignition region or the spark ignition region. Check out. Note that the compression ignition region in the present embodiment is set to a region surrounded by a solid line including the hatched region in FIG. Further, the compression ignition region is divided into a stratified compression ignition region and a middle load region according to the engine load Lo, and the middle load region is divided into a uniform compression ignition region. In the stratified compression ignition region, the fuel injection timing is relatively low in the latter half of the compression stroke. Set it later. In the uniform compression ignition region, the fuel injection timing is set between a relatively early time during the intake stroke after the start of the negative valve overlap period (see FIG. 6).
[0029]
When the operation region is in the compression ignition region, the process proceeds to step S2, and when it is in the spark ignition region, the process proceeds to step S6.
[0030]
In step S2, the throttle valve 9 is fully opened, and then in step S3, a signal for selecting a compression ignition intake cam and a compression ignition exhaust cam is output to the variable valve mechanisms 13a and 13b. The intake valve 6 and the exhaust valve 7 are operated at the valve timing at the time of compression ignition.
[0031]
Next, the process proceeds to step S4, and a negative valve overlap period is set. As shown in FIGS. 4A and 4B, the negative valve overlap period is maximized during low load operation, minimized during medium load operation, and variably set in accordance with the engine load Lo within that range. Is done.
[0032]
Specifically, based on the engine load Lo, the valve overlap period setting table shown in FIG. 5C is referred to with interpolation calculation using the engine load Lo as a parameter, and the variable valve mechanisms 13a and 13b are driven. Output a signal. Then, the VVT mechanisms provided in the variable valve mechanisms 13a and 13b change the rotational phases of the compression ignition intake cam and the compression ignition exhaust cam, respectively, and the intake valve 6 is opened during low load operation. The valve timing IVO is retarded and the valve closing timing EVC of the exhaust valve 7 is advanced to widen the negative valve overlap period. On the other hand, during the medium load operation, the valve opening timing IVO of the intake valve 6 is advanced, and the valve closing timing EVC of the exhaust valve 7 is retarded, thereby narrowing the negative valve overlap period.
[0033]
As shown in FIG. 4B, the cam profiles of the compression ignition intake cam and the compression ignition exhaust cam indicate that the closing timing IVC of the intake valve 6 during the middle load operation is near the intake bottom dead center (BDC). Further, the valve opening timing EVO of the exhaust valve 7 is set to be the expansion bottom dead center (BDC), and is formed by the valve closing timing EVC of the exhaust valve 7 and the valve opening timing IVO of the intake valve 6. The negative valve overlap period is set to a symmetrical position with respect to the exhaust top dead center (TDC). Then, as the engine load shifts from the medium load operation to the low load operation shown in FIG. 5A, the compression ignition intake cam and the compression ignition exhaust cam change their rotational phases symmetrically.
[0034]
As a result, as shown in FIG. 6, as the engine load shifts from the medium load operation to the low load operation, the valve timing of the exhaust valve 7 is advanced, while the valve timing of the intake valve 6 is Is retarded in the direction of symmetry.
[0035]
In this way, by delaying the closing timing of the intake valve 6 at the time of low load operation where the thermal energy of the residual gas is the lowest, volume efficiency is improved by inertia supercharging, and more fresh air is obtained. Can be inhaled. In addition, since the negative valve overlap period at this time is set wide (see FIG. 5 (c)), the amount of residual gas confined in the combustion chamber 3 is large, and intake air heating by the thermal energy of this residual gas is reduced. As a result, good compression ignition performance can be obtained even at a leaner air-fuel ratio.
[0036]
On the other hand, when the residual gas confined in the combustion chamber 3 has a relatively high thermal energy, the negative valve overlap period is narrow during medium load operation, so that intake air heating due to the residual gas thermal energy is suppressed, and the intake valve Since the valve closing timing of 6 is set in the vicinity of the intake bottom dead center (BDC), the volumetric efficiency is reduced and the compression pressure is lowered, so that the self-ignition is suppressed and the occurrence of knocking is avoided and rich mixing is performed. A stable compression ignition combustion can be obtained. As a result, as shown in FIG. 3, the uniform compression ignition region can be expanded from the conventional region indicated by hatching to the high load side.
[0037]
Next, the process proceeds to step S5, the compression ignition fuel injection control is executed, and the routine is exited. In the compression ignition fuel injection control, processing for variably setting the fuel injection amount and the fuel injection timing is performed.
[0038]
As shown in FIG. 5A, the fuel injection amount is controlled so that the air-fuel ratio gradually becomes leaner as the engine load Lo decreases. In the figure, symbol λ0 indicates the stoichiometric air-fuel ratio. The fuel injection timing is set based on the engine speed Ne and the engine load Lo. For example, when the operation region is a low load region, the fuel injection timing is set later than the intake bottom dead center (BDC), that is, after the compression stroke starts. On the other hand, at medium load and low / medium speed, it is set at a relatively early time from when the exhaust valve 7 is closed (at the start of the negative valve overlap period) to the intake bottom dead center (BDC).
[0039]
By injecting fuel from the in-cylinder injector 11 into the combustion chamber 3 at a time later than the intake bottom dead center (BDC), the fuel is stratified in the combustion chamber 3 that is reaching a gas temperature at which compression ignition combustion is possible. The air-fuel mixture is generated locally, and stratified compression ignition combustion with an extremely lean air-fuel mixture becomes possible. On the other hand, by setting the fuel injection timing relatively early after the exhaust valve 7 is closed, a uniform air-fuel mixture can be generated before the gas temperature in the combustion chamber 3 reaches a temperature at which self-ignition is possible. When the temperature reaches the ignition temperature, combustion in which the air-fuel mixture ignites all at once and the flame does not propagate is realized, that is, multipoint ignition combustion (uniform compression ignition combustion) in which an infinite number of ignition plugs are arranged.
[0040]
If it is determined in step S1 that the operation region is in the spark ignition region and the process proceeds to step S6, combustion control by normal spark ignition is executed and the routine is exited. When the process shifts to the spark ignition combustion control, a signal for switching to the spark ignition intake cam and the spark ignition exhaust cam is output to the variable valve mechanisms 13a and 13b. As a result, the valve timing at the time of normal spark ignition of the intake valve 6 and the exhaust valve 7, that is, a positive valve overlap period in which both the valve opens from the end of the exhaust stroke to the beginning of the intake stroke (FIG. 4 (c), FIG. 5 (c) ))). The cam profile of the spark ignition intake cam and the spark ignition exhaust cam is set to a shape that maximizes volumetric efficiency.
[0041]
At the same time, the throttle valve 9 is operated in conjunction with the accelerator pedal 32 (see FIG. 5 (d)), and the fuel injection amount, fuel injection timing, ignition timing, and the like are returned to normal spark ignition control. Since these controls are well-known, explanation here is omitted.
[0042]
【The invention's effect】
As described above, according to the present invention, during the compression ignition operation, as the engine load increases, the negative valve overlap period is narrowed to reduce the intake air heating, and further, the intake valve closing timing is set. By reducing the volumetric efficiency by advancing, self-ignition can be suppressed, and stable compression ignition performance can be obtained in a region where the air-fuel ratio is deeper. As a result, the compression ignition region can be expanded to the high load side. Is possible.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of a four-cycle gasoline engine having a compression ignition region . FIG. 2 is a flowchart showing a combustion control routine. FIG. 3 is an explanatory diagram showing an operation region map. (A) is a low load operation, (b) is a medium load operation, (c) is a high load operation. FIG. 5 is an explanatory diagram showing the relationship between the engine load and each control characteristic. 6] Explanatory diagram showing the relationship between valve timing and in-cylinder pressure characteristics

Claims (3)

4 cycles having a variable valve mechanism capable of forming a negative valve overlap period for closing both the exhaust valve and the intake valve before and after exhaust top dead center, and having a compression ignition region in a part of the operation region In gasoline engines,
In the compression ignition region, as the engine load increases, the negative valve overlap period is narrowed and the closing timing of the intake valve is advanced so that the volumetric efficiency is reduced. .   2. The four-stroke gasoline engine according to claim 1, wherein in the compression ignition region, the closing timing of the intake valve during medium load operation is set near the intake bottom dead center.   3. The four-cycle gasoline engine according to claim 1, wherein in the compression ignition region, the closing timing of the intake valve during low-load operation is retarded to a position exceeding the intake bottom dead center.

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